1. Field of the Invention
The present invention relates to a cornea imaging apparatus for imaging an cornea of the eye by means of directing illumination into an eye being examined and receiving the light reflected from the cornea of the eye being examined.
2. Description of the Related Art
To date, observation of the cornea of the eye, and in particular of the cellular status of the corneal endothelium, has been commonly carried out when determining the presence of ocular disorders, in the course of eye surgery, and similar situations.
A cornea imaging apparatus capable, through a noncontact method, of imaging the cells of the corneal endothelium of the eye under examination during such observation of the cellular status of the corneal endothelium are known in the art. This cornea imaging apparatus is designed to direct a slit beam of illumination light from an optical system onto the cornea of the eye under examination, and receive the light reflected from the cornea in order to image the cells of the corneal endothelium.
However, a problem with such cornea imaging apparatus has been that, owing to the small thickness dimension of the corneal endothelial cells, it is frequently difficult to obtain a sharp focused image of the corneal endothelial cells. Particularly since such cornea imaging apparatus employ a slit beam of illumination light, it is necessary to accurately align the illumination optical system and the imaging optical system at the focal position of the endothelium in the near/far direction with respect to the corneal endothelial cells, in order to avoid the adverse effects of light reflected by the epithelium and parenchyma of the cornea.
Accordingly, there has proposed in the past, for example, in Patent Document 1 (U.S. Pat. No. 5,436,679), an apparatus employing a line sensor for detecting the distribution of intensity of reflected light from the cornea, detecting the focal position of the corneal endothelial cells, and aligning the optical system with it. Specifically, by taking into consideration the distribution characteristics of the intensity of reflected light in the cornea, which is composed of the corneal epithelium, parenchyma, and endothelium, the focal position of the corneal endothelial cells is estimated from the location of the peak in the output values of the line sensor. The corneal endothelial cells are then imaged with the illumination optical system and the imaging optical system aligned with this estimated focal position.
However, a problem with the cornea imaging apparatus of the prior art design like that disclosed in Patent Document 1 is the difficulty of detecting with consistent accuracy the focal position of the corneal endothelial cells. Specifically, since differences in corneal thickness exist among individuals, including for example instances of reduced corneal thickness as a result of corrective surgery for refractive errors or the like, where the cornea is thin it is difficult in practice to detect both the peak of the epithelium and of the endothelium, with the risk of an inability to correctly identify the endothelial focal position. There also exists a risk that, due to the properties of the corneal parenchyma, and in particular to clouding of the parenchyma caused by ocular disease or the like, conflicting levels of reflection by the parenchyma and the endothelium may make it impossible to detect focal position from the peak of the endothelium, or result in erroneous location detection.
Furthermore, even if focal position is successfully detected with good accuracy, since the imaging process is carried out subsequent to alignment with the estimated focal position detected by the line sensor, slight movements of the eye may throw off the focal position, with the risk that a sharp focused image of the endothelium will not be obtained.
In Patent Document 2 (Japanese Pat. No. 2831538) for example, there is disclosed a cornea imaging apparatus constituted so as to move the optical system forward in the direction nearer to the eye under examination; to first detect the focal position with respect to the epithelium; and from the focal position with respect to the epithelium; to then estimate the focal position of the corneal endothelial cells based on anatomical thickness of the cornea. In this cornea imaging apparatus, first, the optical system is moved in the direction nearer to the eye under examination while directing illumination into the eye under examination. Then, after detecting the focal position with respect to the epithelium by means of detected reflected light from the epithelium, the focal position is moved towards the endothelium by a distance depending on the distance separating the endothelium from the epithelium, which distance was established previously on the basis of anatomical thickness of the cornea. The focal position is thereby aligned with the corneal endothelial cells.
However, accurate detection of the focal position of the corneal endothelial cells proved difficult to detect with the cornea imaging apparatus disclosed in Patent Document 2 as well. Specifically, as noted above, there are differences in corneal thickness among individuals, and since the distance between the epithelium and the endothelium will differ according to the patient under examination as well, the correct position will not necessarily be attained simply by moving the optical system from the epithelium towards the endothelium by a preset distance. Moreover, while the cornea imaging apparatus in question is adapted to perform continuous imaging over a wide range in consideration of such individual differences, reliable imaging of the corneal endothelial cells requires carrying out continuous imaging over a wide range able to encompass individual differences, and the imaging will be time-consuming. A resultant problem is that the subject under examination must keep his or her eyes open while being exposed to bright illumination, and thus prolonged imaging time will impose considerable strain on the patient.